The present invention relates to a system and corresponding method for broadcasting system information in a cellular communications network.
In a typical cellular communications system, a geographical area is divided into several xe2x80x9ccellsxe2x80x9d each of which is served by a base station (BS) having a limited radio coverage area. Base stations are in turn connected to a mobile services switching center(s) (MSC) which is, in turn, connected to a landline public switched telephone network (PSTN). Each user (mobile subscriber) in the cellular radio system or network is provided with a portable, pocket, handheld, or car mounted mobile station (e.g., cell phone) which communicates voice information and/or data with a nearby base station which defines the cell in which the mobile station (MS) is located. The MSC with which the base station is in communication switches calls and controls signaling between the mobile station (MS) and other mobile stations in the system or landline telephones in the PSTN.
The wireless communications link (e.g., radio interface) between a mobile station and corresponding base station includes a number of different logical channels which may be separated into two broad categories: traffic channels and control channels. Traffic channels are utilized by a serving base station controller to communicate call data (e.g., voice data) with a particular MS travelling within the cell defined by the base station. Control channels are utilized by the serving base station controller to communicate control data (e.g., system information) necessary to implement the connection of call data with the MS. Exemplary control channels include broadcast channels (BCH), Common Control Channels (CCCH) and Dedicated Control Channels (DCCH). Each of the above three categories of control channels may still further be sub-divided into a number of logical channels for transporting different types of information between the serving base station (BS) and a mobile station (MS). Broadcast control channels, for example, may be used to broadcast general system information about the cell defined by the broadcasting base station to mobile stations located within or near the cell.
In a cellular communications system or network, system information parameters are defined in order to provide network configuration, adaptation, and optimization possibilities. Certain of these parameter values must be known by a mobile station (MS) before a first access attempt can be made by that MS. Therefore, the parameters (cell parameters or system information parameters) are typically constantly broadcast via control channel(s) in every cell of the network in the form of system information messages, and a mobile station (MS) typically must wait until it has successfully received all necessary system information parameters from the BS before performing an access attempt. Exemplary cell parameters (or system information parameters) include cell descriptions, location area identity, neighboring cell description or relation, physical cell parameters, cell traffic load, traffic measurement information and/or core network information. For purposes of simplicity, certain cell or system information parameters herein may be referred to by the reference characters xe2x80x9cMxe2x80x9d, xe2x80x9cNxe2x80x9d, xe2x80x9cRxe2x80x9d, xe2x80x9cSxe2x80x9d, xe2x80x9cTxe2x80x9d, xe2x80x9cXxe2x80x9d, xe2x80x9cYxe2x80x9d, and/or xe2x80x9cZxe2x80x9d.
The amount of system information in today""s cellular systems or networks is significant. The need for a MS to read all such information tends to create delays for the MS at registration for or at its access attempts following a cell change (a cell change is when a MS moves from one cell to another). Furthermore, when an MS is in an idle or standby mode, it typically needs to check the system information at regular time intervals so as to be kept up-to-date regarding the same. This reading of system information by many mobile stations throughout the network requires significant radio resources and reduces battery life of the individual mobile stations as well as the corresponding maximum standby time available to each MS.
A cellular network operator may often choose to use constant values for many of the system information parameters in a group of cells or even throughout the entire network. In such situations, since the MS does not know which system parameters have changed and which have not, it has to re-read all of the system information continuously or at each cell change.
Thus, it will be apparent from above that problems with broadcasting system information in conventional cellular communication networks include the following. First, an MS is forced to reread (e.g., at each cell change) system information parameters that may in fact be identical to earlier read parameters. This is not an efficient use of resources. Second, reading vast amounts of system information requires significant radio resources in an MS and the overall network, and may have a significant adverse impact on battery consumption of the MS. Third, system information parameters may be defined by a particular standard, and thus be difficult to change later in time when new features and/or parameters may be introduced in the system. Fourth, a sequential order of sending/transmitting system information blocks by base stations may be defined by a standard and thus be difficult to change at later point(s) in time when new features and/or parameters are introduced into the system.
One approach to resolving, inter alia, the second problem discussed above is set forth in commonly assigned U.S. Pat. No. 5,404,355, the disclosure of which is hereby incorporated herein by reference. In the ""355 patent, system information elements are grouped and change flags are provided to indicate whether values of information elements have changed. The change flag and information element are transmitted on a control channel. Accordingly, a MS only has to read the system information one time in a given cell provided that the information does not change. However, the system of the ""355 patent has its problems as well. For example, it does not eliminate the need for mobile stations to read all system information parameters when switching cells (i.e., at cell change). Instead, a MS must read all system information any time it locks onto a new control channel (e.g., at cell change).
In view of the above, it will be apparent to those skilled in the art that there exists a need in the art for a system and/or corresponding method which enables one or more of the aforesaid four problems to be addressed. For example, there exist a need in the art for a system and/or method for reducing the need for a mobile station (MS) to re-read system information parameters at cell change when such parameters have, in fact, not changed at all from one cell to another.
In a cellular telecommunications system or network, system information is broadcast from a UTRAN to idle mode and/or connected mode mobile stations in a cell. In essence, the system information may be organized as a tree, including master information block(s) and underlying system information blocks. A master information block, broadcast on a master channel, provides a receiving MS with reference(s) to a number of system information blocks in a cell, including scheduling information for those system information block(s). A system information block(s) groups together system information elements of the same or similar nature. Different system information blocks that are broadcast may have different characteristics, e.g., regarding their repetition rate and/or requirements on MSs to re-read system information blocks. The system information blocks contain actual system information parameters and/or references to other system information block(s) including scheduling information for those system information block(s).
In certain embodiments, tags are broadcast in master information blocks. Tags are each associated with one or more system parameters. For example, a given tag value may be indicative of particular values for three separate system information parameters. In each cell, a base station (BS) transmits or broadcasts currently valid tag values for that cell on a control channel. System information blocks including the system information parameters themselves are in turn broadcast by the base station (BS) in each cell on the same or other control channel(s). When a mobile station (MS) enters a new cell and locks onto a new control channel, it reads the valid tag value(s) in that new cell via the master control channel. If the MS determines that it already has stored and/or is using the system information parameters corresponding to all valid tag values, then there is no need for the MS to read the system information parameters in the new cell at cell change. If, however, the MS determines that it does not have stored certain system information parameters corresponding to valid tag value(s) in the new cell, then the MS reads the necessary system information parameters. Thus, in certain embodiments of this invention, the use in a cell of several tags is provided with each tag including part of the system information; thereby making it possible to change a subset of tags in a cell and thus making it possible for a MS to only have to read the relevant new system information.
As will be appreciated by those skilled in the art, different embodiments of the instant invention may result in one or more of the following advantages. For example, the MS does not have to re-read identical system information each time it changes cells, or while moving in the same cell. This saves radio resources and reduces battery consumption in the MS thereby improving the standby time of the MS. Another advantage is that delay associated with reading all new system information parameters at each cell change may be reduced by re-using cached (i.e. stored) system information parameters when valid tag value(s) do not change at cell change. Only new system information parameters not already being stored by the MS need be read by the MS at cell change in certain embodiments.